An image forming apparatus such as a copying machine, a printer, or a facsimile machine includes, for example, (i) a toner density sensor for reading the toner density and (ii) a mispositioning sensor for reading the position of an image to detect mispositioning. Such a toner density sensor and mispositioning sensor are each typically a reflective sensor including a light emitting element and a light receiving element.
A reflective sensor is arranged such that a light emitting element irradiates a detection target object with light and that a light receiving element reads reflected light from that detection target object. The light receiving element generates a photocurrent corresponding to the intensity of light that the light receiving element has read. The reflective sensor detects such a photocurrent in terms of a voltage value. The image forming apparatus then forms a toner image of the detection target object on a recording medium or an intermediate transfer member. This allows a reflective sensor to, on the basis of a photocurrent generated (voltage detected), detect the toner density or detect the position of a toner image to detect mispositioning.
Conventional techniques detect toner densities in different manners for different colors having respective characteristics in terms of light absorption and light diffusion. Specifically, conventional techniques detect (i) the density of a black toner on the basis of a voltage detected as a result of reading regularly reflected light and (ii) the density of any color toner such as a cyan toner, a magenta toner, or a yellow toner on the basis of a voltage detected as a result of reading diffuse-reflected light. Conventional techniques detect mispositioning on the basis of either (i) a decrease in the amount of regularly reflected light received or (ii) an increase in the amount of diffuse-reflected light received.
Patent Literature 1, for example, discloses an image forming apparatus including an image detecting device capable of detecting information on the position of an image highly accurately. This image forming apparatus includes (i) a lighting lens for directing light from a light source onto an image on a recording material being carried and (ii) an imaging lens for forming that image on a light receiving element. The image detecting device uses the two lenses to narrow the imaging spot on the light receiving element for minimum spherical aberration in order to detect information on the position of an image highly accurately.
Patent Literature 2 discloses a technique of detecting the amount of mispositioning of an image with use of a reflective sensor for receiving regularly reflected light. This technique uses (i) a first aperture for narrowing light from a lighting source section which light is directed onto a toner image on an image holding member being carried and (ii) a second aperture for narrowing reflected light regularly reflected by the toner image and then received by a light receiving section. The technique sets an optimal diameter for each of the first and second apertures for a balance between robustness and influence of diffused light.